is there really no replacement for displacement?...

Well you asked for opinions, so here goes. I have all the respect in the world for /6s, but I have yanked them out of the two cars I was keeping.

70 Purple Barracuda I had years ago, was a 3 speed slant, put a warmed up 318 and 904, 8 3/4 in it. Now, 22 years later, it would be really cool if it was original, but not then. The next owner made it a full blown 340 4 speed Cuda clone.

My 68 Barracuda was a /6 car, and I put a 340 in its place. In fact, I just like messing with it, and I have my 3rd motor in it.

The only car I left as a /6, was my daughters 67 FB Barracuda, her high school car. I put an aluminum slant in it for a while, until she blew the head gasket.